My original email:
"I’m collaborating on an LID maintenance manual for rain gardens, swales, planters, and vegetated filter strips. In my narrative, I’m recommending against using crushed aggregate [note: I meant crushed concrete, which was at least, in the subject of the email (crushed concrete & pH)] as a check dam because of the pH problems it causes to aquatic life. I’ve dug around on the internet for an hour now… Anybody have a scientific paper that I can reference to back up this claim?"
My second email in response to the question, "Where did you get the idea that crushed concrete affects pH?":
In my recent CESCL (Certified Erosion and Sediment Control Lead) erosion inspector certification class. Washington State’s Department of Ecology regulates water coming from a construction site for pH. If a sample is NOT between 6.5 and 8.5, then the contractor is required to adjust their activities. While I understand that in many places in the country, adjusting pH might be good, our group is writing this O&M manual for landscape contractors in the Pacific NW, where endangered salmon will be impacted by a change in pH.
We’re assuming that most landscape contractors are the “get ‘er done” kind of folks who don’t want a lot of details, but just want to be told a rule of thumb that will protect their recreational fishing time. That’s why I’m proposing that we suggest NO crushed concrete be used as check dams, even though some folks have said that in time, the cement in concrete stabilizes and pH isn’t a problem probably. (The Iowa study that was posted here by others showed a decrease in pH over a year’s time, but that decrease never dipped below 10.) The landscape contractors may not want to take the time to figure out whether it’s old or new, and in Portland region alone, there are already hundreds of bioretention facilities. With Oregon’s new MS4 permits, there will be thousands of bioretention facilities in the not too distant future. If we used crushed concrete in all of them, the watershed scale effects could be significant. Best to apply the precautionary principle (http://en.wikipedia.org/wiki/Precautionary_principle) than harm the watershed in the very facility designed to protect it!
My comments today:
So, I did make a mistake in the text of my email and got a lot of responses on the effect of crushed aggregate instead of crushed aggregate, so I'm breaking this out by crushed concrete info and crushed aggreagate.
And, the winning citation is...
Thanks to Richard Stanford, Simon Gruber,
http://www.iowadot.gov/operationsresearch/reports/reports_pdf/mlr/reports/mlr9604.pdf which indicates that yes, indeed, over the course of a year's worth of intense monitoring, crushed aggregate used in road base does raise pH. It started above 12 and never dropping below 10, but this dip is believed to be an error in the pH meter, and we could really say that it never dipped below 11.5. Unfortunately, they didn't include an analysis of the aggregate that was used in the concrete, but fortunately, they appeared to have field compared the recycled concrete against the traditional, crushed aggregate base. The whole study was initiated because they measured higher pH from projects with recycled concrete base than from road projects with crushed aggregate. They were motivated to investigate the recycled concrete base projects after observing clogged pipes & impacted vegetation downstream.
Responses Received regarding CRUSHED CONCRETE
I'm not positive, but I believe the issue is with concrete mix and readymix - not cured concrete. Once cured, there should be no leaching/acidity issue. I remember looking at this a while ago - I'll poke around the web again and see if I find something.
Maria, Here are some links. Only the first one appears to be peer reviewed but I expect there are others out there. I searched “recycled concrete pH impacts” and one other search. I’ll be interested to see what else you find.
Peer reviewed paper on using recycled concrete to deliberately counter low pH “ This study aims to evaluate recycled concrete as a reactive material in permeable reactive barriers (PRBs) for the remediation of acidic groundwater…”